Bimaleimides may be used as raw materials for the manufacture of polyaddition and polymerization products. For example, U.S. Pat. No. 3,658,764 discloses polyaddition products produced from unsaturated bismaleimides and amines; U.S. Pat. No. 3,741,942 describes the manufacture of polyaddition products from bismaleimides and organic dithiols; U.S. Pat. Nos. 4,038,251 and 4,065,433 describe the reaction of bismaleimides with polyhydric phenols and polyhydric alcohols in the presence of a catalyst to form the polyaddition products; and U.S. Pat. No. 4,100,140 describes polyaddition products prepared from bismaleimides and alkenylphenols or alkenylphenol ethers.
Because of their thermal stability and good mechanical properties, polyaddition products produced from bismaleimides may be used in various prepreg applications. One such polyaddition product, supplied as a powder, is the reaction product of bismaleimidodiphenyl methane and methylene dianiline. The powder polyaddition product may then be dissolved in a high boiling solvent, such as N-methylpyrrolidone, for prepreg use. The stability of such a solution however is limited due to rapidly occurring precipitation and viscosity increase upon standing. Therefore, formulation of the solution must occur immediately prior to use.
Improvements have been noted in the polyaddition products and solutions disclosed in U.S. Pat. Nos. 5,189,128 and 5,637,387. U.S. Pat. No. 5,189,128 discloses a polyaddition product produced from the reaction of polymaleimide and alkenylphenol or alkenylphenol ether in the presence of specific molar amounts of basic catalyst for certain reaction times and at certain reaction temperatures. The resulting product exhibits improved solubility in the lower boiling solvent methyl ethyl ketone and improved storage stability as evidenced by an absence of precipitation. A further improvement in storage stability is described in U.S. Pat. No. 5,637,387 where phenothiazine or hydroquinone is added after the polymaleimide has reacted with the alkenylphenol or alkenylphenol ether.
It is an object of the present invention to provide an improved polyaddition product composition which exhibits resistance to acids, and in particular, phosphoric acid, without any adverse impact on the thermal and mechanical properties of the prepreg products made therefrom.